The present invention relates to spectrometer apparatus for separating light radiation into a plurality of spectral bands and, more particularly, to a dispersive optical spectrometer for efficiently dispersing spectral radiation for performing multielement spectral analysis.
Optical spectrometers have been known for quite some time. These devices are based on a dispersive element, which may be a prism or a diffraction grating, such as a plane or concave diffraction grating. A typical conventional optical arrangement, such as an Ebert or Czerny-Turner mounting, is comprised of the following components: (1) a single entrance aperture to admit radiation, (2) a collimating component to render the admitted radiation into a parallel bundle of rays, (3) a dispersion device such as a prism or diffraction grating, (4) a focusing component to focus the dispersed radiation as images along the plane known as the focal plane, and (5) detection means upon which the dispersed spectrum is focused and which is some manner transduces the optical information striking it into some usable form. In systems which employ a concave diffraction grating, the collimating and focusing components are unnecessary.
Various types of detectors may be used, such as, for example, film, photodiode circuits or optoelectronic multichannel vidicon detectors. One such vidicon may be an optical multichannel analyzer (OMA) coupled wih a silicon intensified target (SIT) detector described in catalog number T388-15M-5/78-CP published by Princeton Applied Research Corp., Princeton, N.J.
These dispersive optical spectrometers employ a single entrance aperture so that the spectral information is dispersed in the exit focal plane as a single band of radiation along a single direction of dispersion, for example a single horizontal band of radiation. With this arrangement, the position of the various radiational components along the focal plane defines the wave length of the various radiational components which may be present.
This optical arrangement is perfectly satisfactory for use with photographic detection because the photographic plates may be fabricated in lengths sufficient to cover the spectral region of interest, from the ultraviolet to the near infrared. The advent of optoelectronic detectors, such as the vidicon and other electronic image detectors, has demonstrated that these devices offer some distinct advantages for the spatial detection of dispersed radiation. Their major limitation at present seems to be the limited amount of dispersed spectral information which can be simultaneously imaged on the image or light detector. When such detectors are used with a conventional dispersive spectrometer as described above, only a limited spectral region can be accessed by the image detector simultaneously. This limited spectral region, or window, is defined by: EQU W=R.sub.d D (1)
where W is the extent of spectral region or the window width, R.sub.d is the reciprocal linear dispersion of the spectrometer and D is the diameter of the light sensitive face plate of the image tube. Thus the window width may be increased by either increasing R.sub.d or D. Because of the nature of the image detectors, the prospect for increasing D is remote. If R.sub.d is increased, by changing the focal length of the collimator/focusing elements or by employing a more coarsely ruled diffraction grating, more spectral information may be compressed across the face plate of the image detector. However, this modification results in poorer resolution.
The resolution of the system determines how close any two spectral components may be and still be recognized as separate, distinct spectral components. Thus, the use of an image detector with a conventional single aperture dispersive spectrometer results in a compromise between window widths and resolution. For atomic spectroscopy one should ideally cover the entire spectral region of interest from the ultraviolet to the near infrared at high resolution to cover the maximum spectral region under conditions where as many as possible of these spectral components are recognizable as distinct, separate spectral components which do not overlap adjacent spectral components.
To overcome this limitation of conventional one-dimensional dispersive systems, an optical arrangement known as an echelle system has been employed in conjunction with image detectors. Such systems use a specially, more coarsely ruled grating, known as an echelle grating, in order to provide high dispersion. In addition to the echelle grating, another dispersing device such as a prism is arranged to disperse the radiation orthogonal to the direction of dispersion of the echelle grating. Such a crossed dispersion optical arrangement results in a two-dimensional array of spectral information because the prism disperses all the various diffraction orders produced by the echelle grating. This system, however, employes complex optics and does not display the spectrum in parallel bands. Furthermore, the bands are nonuniform in size which makes detection difficult and inefficient since large portions of the target are not utilized. Also, separation between parallel bands is not uniform. Resolution also varies across the spectrum in this type of system. In order to utilize the optoelectronic detector apparatus, the nonparallel display of bands calls for a complex computer controlled detector system and constitutes an inherently inefficient use of the two-dimensional vidicon target. This system also is notorious for its high stray light levels, a direct result of the optical design and configuration.